This is the kind of thing that it would be good to ask your nearest USDA Cooperative Extension Service branch office about. With it perhaps being necessary to do some sampling and testing to zero in on what exactly is happening. But I don't know what the Extension response to the pandemic has been, if they have minimized their operations or what.

So I sent off soil samples to a lab as well as the plant to the University of Florida for testing.

What they found is that the plant has Amarilla Root Rot.

If they said anything about what to do in the report or you looked it up afterwards then you may know control involves digging around the infested specimen and pulling the rhizomorphs off it on a recurring basis. Because it's a fungus in the soil that uses rhizomorphs to come up into the air and attack the stems of a targeted tree or shrub - like ground termites making tubes to get at wooden house walls.

one of the base problems is the monoculture of the planting ...

perhaps you ought to start replacing plants.. with something that isnt affected by such .. if there are such things ...

i mean really.. if you have to basically dig out the plant to rid it of underground root problems.;.. seems like your most the way to just removing them ...

keep in mind.. the old cost/benefit ratio ... dont go spending hundreds or thousands trying to save them ... and do include your own labor in said ratio ... even if you dont pay yourself ...

ken

What plant could I put in that will grow to a 8’ hedge line the rest that are still ok?

the doctor at the university said I basically have to dig them all up and burn them and can’t plant any viburnum there every again!

Here is a pretty detailed listing of plants that are known to be resistant - not immune - to armillaria. While this is focused on California, I'm sure the Florida extension service publishes a similar listing.